Vehicles having a hybrid drive structure have an internal combustion engine and another drive unit which may be designed as an electric motor or as a hydraulic motor. The drive torque may thus be provided either by both drive units or by only one drive unit while the hybrid vehicle is in driving operation.
The internal combustion engine is started during electric driving operation of the hybrid vehicle by a connection to a drive train, which is driven by the electric motor. The need for starting the internal combustion engine arises, for example, when the driver wants more power than the electric motor is able to deliver or when the energy level of an energy storage which supplies electric power to the electric motor has dropped too much.
A starting torque or a starting power required for starting the internal combustion engine must be provided by the additional drive unit, for example, the electric motor, this power being transmitted to the internal combustion engine via a slipping clutch. A clutch operated in slipping mode has inaccuracies. Such inaccuracies include varying coefficients of friction of the clutch linings due to wear or changes in temperature, hydraulic or mechanical deviations in the operating system of the clutches or in hystereses, signal transit times and aging phenomena during the activation. The clutch torque transmitted by the clutch operated in slipping mode is not known exactly and cannot be adjusted accurately. This prevents accurate compensation of the starting torque or the starting power of the internal combustion engine by the additional drive unit. As a result, the uncompensated component acts as interference upon the drive train, which excites rotational vibrations and is perceived as jerking of the vehicle, which has a negative effect on the driving comfort.